1. Field of the Invention
The invention relates to a method and apparatus for recording and/or reading audio or other information in the form of a track of optically detectable pits and land areas in a record carrier, the information being in the form of sequences of binary data bits representing the changes in successive quantized samples of the information. The stream of data bits is divided into consecutive frames each of which comprises consecutive symbol blocks of m data bits, each data symbol block being encoded for recording as a symbol block of n.sub.1 +n.sub.2 channel bits, (n.sub.1 +n.sub.2)&gt;m, where n.sub.1 is the number of symbol information channel bits and n2 is a number of channel "merging" bits which are added to assure that a change in binary value persists long enough to result in at least a minimum pit or land run length. The invention also relates to a record carrier having information recorded thereon in accordance with the aforesaid method.
2. Description of the Related Art
Digital recording of information on an optical disc record carrier by means of a laser beam is described in the publication "Philips Technical Review", Vol. 40, 1982, No. 6, particularly the articles "Compact Disc: System Aspects and Modulation" by J. P. J. Heemskerk and K. A. Schouhamer Immink at pages 157-164; and the article "Compact Disc Digital Audio System" by Messrs. Carasso, Peek and Sinjou at pages 151-155. For a more complete understanding of digital disc recording, reference may be made to that publication. In that method, the analog audio signal is sampled at a rate of 44.1 kH.sub.z and the samples are quantized and digitally encoded into 32 bits per sample. The 32 bits corresponding to each sample are grouped into "symbol" blocks of 8 bits each, and these are encoded by an "Eight to Fourteen" (EFM) modulator into new symbol blocks of 14 channel bits such that any "0" or "1" bit occurs at least twice in succession. This assures that the run length of each pit and land produced in the record carrier by the channel bits will extend for at least 3 channel bits, which substantially increases the recorded information capacity of the disc record carrier. In order to maintain at least the minimum run length when the channel bits of successive symbols are merged into a single channel bit stream, at least two additional "merging bits" are added to the channel bits for each symbol. As a result of this, however, the digital sum value (DSV) of the channel bits of successive symbols may become appreciable, resulting in production of a significant d.c. or average value of the channel bit signals. This is undersirable because, as pointed out in the first of the above-identified articles, changes in average signal intensity due to soiling of the disc can result in read-out errors if such changes reduce the signal below the read-out decision level. These low-frequency components of the signal can be eliminated by filtering, but this is feasible only if the information signal itself contains no low frequency components. Accordingly, a third merging bit is added to the channel bits for each symbol and which is selected so as to minimize the DSV of the channel bits of successive symbols.
Generally, as pointed out in the second of the above-identified articles, the symbol blocks of data bits are interleaved and provided with additional error correcting parity bits prior to being translated into channel bits.
It has been found that under certain conditions, despite the addition of merging bits to minimize the d.c. unbalance (or DSV) of the channel bits, the DSV may become sufficiently significant to adversely affect read-out of the channel bits.